Is it forbidden for a questioner to tell his brother what the lying brother said about the (other) brothers?

If a brother denies that his siblings helped him, and that is not true, then he has combined lying with denying kindness. His incitement of his sister against her husband is a reprehensible act, and its perpetrator is not of the Muslims. The brother must be prevented from speaking ill of his siblings, and he should be advised gently and kindly. He should also strive to earn a living so that Allah may make him independent of people. Conveying his words to another brother is backbiting that causes corruption, unless it serves a legitimate Islamic interest.